The present invention relates to coating removal systems and, more particularly, to a coating removal system having a solid particle nozzle with a detector for detecting particle flow and associated method.
The use of composite structures manufactured, for example, of graphite epoxy or other reinforced plastic materials has become increasingly common. Reinforced composite materials, including graphite epoxy materials, are extensively employed for surface structures in aircraft and automobile construction. These structures are often painted for a variety of reasons, including aesthetics, identification, and camouflage. However, such painted surfaces deteriorate under the action of weather and the mechanical forces to which they are subjected, thus requiring periodic removal and replacement of the paint.
The removal of paint and/or other coating from the large and often delicate surfaces, as typically found on aircraft and automobiles, is a difficult process which can be compounded by topological anomalies such as rivets or even complex curvature. Techniques such as particle medium blasting (PMB) and mechanical grinding, that are sufficiently energetic to remove paint by themselves, tend to damage composite materials. Paint removal with chemical agents is likewise unsatisfactory since the chemicals tend to attack the organic binder in the composite as well as the paint. Further, high temperature paint removal methods may produce deleterious effects in heat-sensitive composites. Other than labor-intensive hand sanding, one effective method of removing materials such as paint, radar absorbing material (RAM), other coating adhesives, and excess resin from a composite structure comprises using both radiant energy and a particle stream to remove the material or coating adhering to the surface of the substrate.
According to this method of removing a coating from a substrate, the coating is first heated with a pulsed radiant energy source such that the coating is pyrolized and vaporized from the surface. Pyrolisis of the coating reduces the cohesion of the material to itself and its adhesion to the underlying substrate. Any remaining pyrolized coating is able to be removed by a relatively low-power particle stream since this pyrolized coating does not adhere well to the surface of the substrate. Typically the preferred particle stream comprises CO2 pellets that act both as an abrasive agent for removing pyrolized coating and a cooling agent for cooling the underlying substrate. Thus, the pulsed radiant energy source generally accomplishes most of the coating removal while the particle stream is useful for removing any residue as well as for cooling the substrate.
In a typical form, the coating removal apparatus comprises a central radiant energy source having an adjacent particle nozzle aimed so as to direct the particle stream alongside and slightly behind the radiant energy source relative to the direction of movement of the radiant energy source with respect to the substrate. The radiant energy source provides intense repetitive flashes of broadband (ranging from infrared to ultraviolet) radiation to pyrolize and remove the coating from the substrate. The particle stream is then directed at the remaining pyrolized coating such that the still-hot pyrolized coating is almost immediately removed from the surface of the substrate. A vacuum system is also generally provided adjacent the radiant energy source for collecting the waste removed from the substrate.
The particle stream may comprise, for example, carbon dioxide pellets suitable for removing the residue of the ablated coating from the substrate. Usually, it is desirable for the particle stream to be at a temperature well below the ambient temperature in order to quickly cool the substrate such that the substrate does not sustain heat damage. Generally, the particle stream is delivered from a remote source to the nozzle through a duct or feed line, where the nozzle is configured to provide the desired pattern or footprint of the particles exiting the nozzle for optimizing the removal effect of the particles. However, where the nozzle outlet is shaped as, for example, an elongated rectangle, the minor width may be just sufficient for the pellets to flow through. Occasionally, such a nozzle may become clogged from the pellets supplied from the source. In addition, condensing moisture about the outlet of the nozzle may also cause the nozzle to become clogged.
When the nozzle becomes clogged, the cessation of the flow of particles may result in several detrimental effects. For example, the pellet source may continue to produce the pellets and attempt to deliver the pellets to the nozzle, thereby possibly damaging the source if the clog is not expediently discovered and the nozzle unclogged. Further, the radiant energy source may continue to pyrolize the coating without having the pellets flowing from the nozzle to remove the pyrolized coating and provide the necessary cooling for the substrate, thereby possibly leading to heat damage of the substrate. Heat damage to the substrate may result from either the absence of the cooling effect of the pellets resulting from the clogged nozzle and/or the heat imparted by a subsequent pass of the coating removal system, once the nozzle has been unclogged, over the portion of the substrate already having the coating pyrolized in the previous pass of the coating removal system. Current coating removal systems of the radiant energy/particle stream type utilize, for instance, thermocouples in the nozzle feed duct to sense and detect pellet flow in the duct. However, the thermocouples are typically placed close to the pellet source and generally have a slow response time, thereby resulting in a delay in detecting loss of pellet flow due to blockage of the nozzle and/or the feed duct between the thermocouples and the nozzle outlet. Thus, there exists a need for an effective device and method for short response time detection of a clogged nozzle outlet in a radiant energy/particle stream coating removal system in order to prevent possible damage to the substrate and/or the apparatus. The detection system is preferably simple, readily implemented, and capable of reliably indicating the status of the pellet flow at the outlet of the nozzle.
The above and other needs are met by the present invention which, in one embodiment, provides an apparatus for removing a coating from a substrate comprising a nozzle having an outlet and adapted to direct a particle stream therethrough at a predetermined flow rate, a signal source for emitting a signal capable of traversing the particle stream, and a signal sensor positioned to detect the signal emitted by the signal source once the signal has passed through the particle stream. The particle stream is directed from the outlet of the nozzle toward a coating on a substrate to remove the coating from the substrate. The signal sensor is adapted to detect an intensity of the signal emitted by the signal source, once the signal has passed through the particle stream, such that subsequent changes in the intensity of the signal that are detected by the signal sensor indicate a change in the flow rate of the particle stream.
According to one advantageous embodiment of the present invention, the signal source may be, for example, a light emitting diode, a laser, an incandescent lamp, a gas discharge lamp, or the like that is capable of emitting light comprising at least one wavelength. Accordingly, the signal sensor may be, for example, a photodiode, a photomultiplier, a bolometer, or the like capable of detecting the at least one wavelength of light emitted by the signal source. To further facilitate removal of the coating, the apparatus may further include a radiant energy source disposed adjacent the nozzle, wherein the radiant energy source irradiates a target area of the coating with a quantity of energy sufficient to at least pyrolize the coating.
Since the radiant energy source exposes the coating to intense, repetitive flashes of broadband (infrared to ultraviolet) radiation to condition the coating for removal by the particle stream, and since the signal source and sensor comprise an optical detection system in some embodiments of the present invention, the signal source and the signal sensor are preferably configured such that interference from the radiant energy source is minimized. In addition, since the signal source and sensor are exposed to a harsh environment about the outlet of the nozzle, embodiments of the present invention further include a shielding device for shielding each of the signal source and the signal sensor from, for instance, the particle stream and/or condensing water vapor. Typically, the particle stream is comprised of carbon dioxide pellets and the signal source and the signal sensor are disposed either within or externally to the nozzle adjacent to the outlet.
A further advantageous aspect of the present invention comprises a method of monitoring a particle flow in an apparatus used for removing a coating from a substrate. First, a particle stream having a predetermined flow rate is flowed through a nozzle having an outlet. The particle stream is directed from the outlet of the nozzle toward a coating on the substrate for removing the coating therefrom. As the particle stream flows through the nozzle, a signal is emitted from a signal source such that the signal traverses the particle stream. The signal is then detected with a signal sensor once the signal has traversed the particle stream. In some particularly advantageous embodiments, detecting the signal comprises detecting an intensity of the signal at the signal sensor which corresponds to a predetermined flow rate of the particle stream such that subsequent changes in the intensity of the signal at the signal sensor indicates a change in the flow rate of the particle stream from the predetermined flow rate. In some instances, the particle stream comprises, for example, carbon dioxide pellets.
In a particularly advantageous embodiment, the signal source and the signal sensor comprise an optical detection system wherein the emitting step comprises emitting a light comprising at least one wavelength from the signal source and the detecting step comprises detecting the at least one wavelength of light emitted from the signal source with the signal sensor. The emitting and detecting steps further preferably occur adjacent to the outlet of the nozzle and either within or externally thereto. Embodiments of the method according to the present invention may further include the step of shielding each of the signal source and the signal sensor with a shielding device during the flowing step, wherein the shielding device may be configured to direct a gas purge flow across each of the signal source and the signal sensor.
Thus, embodiments of the device and method according to the present invention are capable of detecting a reduced flow or a blockage of the particle stream about the outlet of the nozzle and transmitting this information to the device control system with a short response time, thereby reducing the possible damage to the substrate and/or other detrimental effects resulting from an abnormally low flow of the particle stream. Since the signal source and the signal sensor may be readily implemented in existing configurations of coating removal systems, embodiments of the present invention are relatively simple, readily implemented, and capable of reliably indicating the status of the particle stream flow at the outlet of the nozzle.